The present proposal focuses on an analysis of positive and negative behavioral contrast in multiple schedules. Previous research has primarily described the role of reinforcement density and response rate reduction as underlying mechanisms of behavioral contrast. The proposed experiments will focus on the development of a procedure for an assessment of reinforcement "preference" as a necessary condition for the occurrence of behavioral contrast. Specifically, the first experiment will examine behavior maintained by signaled (.5 sec) electrical brain stimulation under a variety of schedule values. In addition, the preference for signaled vs. unsignaled electrical brain stimulation on these schedules will be examined. The purpose is to derive schedules in which signaled vs. unsignaled electrical brain stimulation produces similar or identical response rate characteristics, yet differential preference. The second experiment is a replication of the contrast paradigm which we have employed in this laboratory with the modification that signaled electrical brain stimulation will serve as the reinforcer instead of food. The manipulations in the multiple schedule which should produce contrast are as follows: a shift from a variable interval schedule to extinction, and a shift from signaled to unsignaled electrical brain stimulation. The third experiment which is dependent upon the results of the first two experiments is designed to make differential predictions concerning the roles of response rate reduction and preference as mechanisms of behavioral contrast.